Metallic nanostructures self-assemble through the evolution of material systems toward states of thermodynamic equilibrium. The difference between the free energy of the system in the initial and final states produces a force that drives the transformation of state. However, the system may also have to first climb an energy barrier before a spontaneous reaction can occur. A reaction can also go through several metastable states which each have excess free energy barriers that can stop the reaction before the thermodynamic minimum can be reached. As a result of the complex physics of metal systems there are numerous structures that can be realized. For instance, from a binary metal melt of iron and carbon, there are innumerable varieties of steel that can be produced through variations in cooling rates and the fraction of carbon to iron. The numerous types of transformations from one nanostructure morphology to another are found throughout the solidification process.
Self-assembly in metal and alloy systems is described by kinetics, or the evolution of a system toward a state of equilibrium. Excess free energy (ΔG) above equilibrium can be due to any number of physical or thermodynamical variables e.g. temperature, pressure, chemical composition in binary (or m-ary) systems, electrical potential or gravitational potential. Therefore if a system is initially in equilibrium, an instantaneous change in a thermodynamic variable causes the system to evolve toward a new state of equilibrium. Since the system has been removed from a state of equilibrium there is a change in free energy which drives the reaction. Change in any variable can be associated with a “driving force” that pushes the system towards a state of equilibrium. Transformation between states, or phases, of matter is a function of various state variables such as temperature, pressure or composition. A change in the values of the variables can change the state of the material. For a material to be in a certain phase (in thermodynamic equilibrium) that phase must also have a Gibbs free energy, G that is lower than the energy of any other phases. At the boundaries of the phases (e.g., a ice-liquid boundary) multiple adjacent phases can coexist and have equal free energies. For spherical and rounded surfaces, small radii of curvature can significantly increase free energy (referred to as the Gibbs-Thomson effect) and significantly affect the solubility of the nanostructure in the surrounding material.
The term self-assembly describes the automatic and autonomous transformation from one state or configuration into another. Self-assembly offers the possibility of fabricating materials, structures, and devices with less effort and complexity than by traditional fabrication methods. This is especially appealing in the field of nanostructure fabrication, where with the continually decreasing feature sizes, the cost of traditional fabrication equipment (e.g. electron beam pattern generators, focused ion beam tools, x-ray lithography, extreme UV lithography systems) is becoming increasingly costly. Developing processes that self-assemble with adequate control, precision, and repeatability has great potential to reduce manufacturing costs of current conventional fabrication processes used in the fabrication of integrated circuits and other integrated devices [e.g. micro electro mechanical systems (MEMS), BioMEMS, Microflips, Lab-on-a-chip].
The ability to securely attach nanowires at desired locations has been quite limited and generally unsatisfactory for practical applications. One class of approaches has been to use mechanical manipulation or microfluidics to position a nanowire or nanotube near a surface followed by the application of an electric field or electron beam to attach the object. A second class of approach is to selectively grow nanowires on chemically patterned surfaces. Nanowires can be grown selectively from catalyst nanoparticles by plasma enhanced chemical vapor deposition. However, the required positioning of the nanoparticles at selected locations can be quite difficult due to the small size of the particles. Also, PECVD and other chemical vapor deposition (CVD) methods are usually performed at high temperatures that can damage the substrate material.